The present invention concerns a capillary evaporator for a two-phase loop for transferring energy between a hot source and a cold source, of the type that includes a) a porous material enclosure having an inlet for a heat-conducting fluid in the liquid state, and b) a jacket in which said enclosure is placed to define around the latter a chamber for collecting said fluid in the vapor state, said jacket having an outlet via which the vapor collected by said chamber is removed.
An evaporator of the above kind is known from French patent application No. 94 09459 filed Jul. 29, 1994 by the applicant. Evaporators of the above kind are part of two-phase loops such as that shown in FIG. 1 of the appended drawings, which is used to transfer thermal energy from a "hot source" area A to a "cold source" area B at a lower temperature. The loop takes the form of a closed circuit in which flows a heat-conducting fluid that can be water, ammonia, "Freon", etc, depending on the working temperatures. The circuit includes "capillary" evaporators 1, 1', . . . connected in parallel, condensers 2 also connected in parallel (or in series-parallel), a vapor flow pipe 3 and a liquid flow pipe 4. The direction of flow of the fluid is indicated by the arrows 5. An isolator 6 can be placed at the entry of each evaporator to prevent accidental return flow of vapor into the pipe 4. A supercooler 7 is placed on the pipe 4 to condense any vapor that is inadvertently not totally condensed at the outlet from the set of condensers 2 and to lower the temperature as a safety measure against the temperature locally reaching the saturation temperature leading to generation of bubbles of vapor on the upstream side of the evaporators.
The working temperature of the loop is controlled by a two-phase pressurizer storage container 8 mounted on the pipe 4. This storage container is controlled thermally (by means that are not shown) to control the evaporation temperature.
With this type of loop it is possible in most cases to control the set point temperature for the hot source A to an accuracy better than 1.degree., regardless of the power variations that the loop undergoes at the evaporators or condensers. The hot source can be equipment generating heat and installed on a spacecraft or on the ground, for example, the loop maintaining the temperature of the equipment at a value compatible with its correct operation.
The maximal power that can be conveyed is conditioned by the maximum pressure rise that the capillary evaporators can produce and by the total head losses of the circuit for the maximal power in question. As described in the aforementioned French patent application, with ammonia pressure rises in the order of 5000 Pa can be achieved.
FIGS. 2 and 3 show an evaporator 1 suitable for use in the loop from FIG. 1. It is described in the document "Capillary pumped loop technology development" by J. Kroliczek and R. McIntosh, ICES conference, LONG BEACH (Calif.), 1987. Evaporators of the above type are sold by the American company OAO.
The evaporator 1 includes a metal tubular jacket 9 that is a good conductor of heat with an inlet 10 at one end and an outlet 11 at the opposite end. A cylindrical enclosure 12 with porous material walls is held coaxially inside the jacket 9 by spacers 13 (see FIG. 3).
The porous material, known as the "capillary wick", can be any material having substantially homogeneous pores of appropriate size, for example sintered metallic or plastics (polyethylene) or ceramic materials.
As explained in the aforementioned French patent application, to which reference should be had for more detailed information, in normal operation the space 14 inside the enclosure 12 is filled with the heat-conducting fluid in the liquid state and the annular chamber 15 collects the vapor of this liquid which forms in the chamber due to the effect of the heat generated by the hot source A. The pressure of the vapor is higher than the pressure of the liquid, which enables flow of the heat-conducting fluid in the loop and removal of the heat conveyed towards the cold source B. The power of the installation is increased by disposing a plurality of evaporators in parallel, as shown in FIG. 1.
However, the heat-conducting fluid that flows in the loop is virtually never pure and often contains gases that cannot be condensed in the loop, such as hydrogen. This gas can result from decomposition of the heat-conducting fluid when the latter is ammonia, for example. It can also result from chemical reactions between the ammonia and metallic parts of the loop made of aluminum, for example. In conditions of very low gravity, this incondensible gas can collect in a pocket 16 at the bottom of the enclosure 12, as shown in FIG. 2.
The space 14 inside the enclosure 12 can also contain bubbles 17 of uncondensed vapor of the heat-conducting fluid. This can cause local blocking of the flow of this fluid and therefore thermal runaway of the loop. If a portion of the capillary material constituting the wall of the enclosure 12, subject to the heat flow from the hot source A, is no longer directly supplied with the liquid from the interior of the enclosure, because of a pocket 16 of uncondensed or incondensible vapor or gas, the liquid contained in this portion of the capillary material evaporates quickly. A "punch-through" 18 appears in the enclosure 12 and the pressurized vapor then instantaneously fills the space 14 inside the enclosure 12, which blocks the flow of the heat-conducting fluid.
FIG. 4 is a schematic representation of a different type of evaporator, as described in the document "Method of increase the evaporation reliability for loop heat pipes and capillary pumped loops" by E. Yu. Kotliarov, G. P. Serov, ICES conference, Colorado Springs, USA, 1994. Evaporators of the above type are sold by the Russian company Lavotchkin.
In FIG. 4 and subsequent figures of the appended drawings reference numbers identical to references used in FIGS. 1 through 3 indicate members or units that are identical or similar.
The FIG. 4 evaporator differs from that of FIGS. 2 and 3 in that it incorporates a buffer storage container 19 at the entry of the evaporator proper, which includes a jacket 9 and a porous material enclosure 12 similar to those of the evaporator from FIG. 2. The evaporator further includes a solid wall tube 20 passing axially through the pressurizer storage container 19 and the enclosure 12, this tube discharging at a point near the bottom of the enclosure.
If the heat-conducting fluid arriving via the inlet 10 of the tube contains incondensible bubbles 17 of gas or 17' of vapor, the bubbles pass through the tube 20 and return "countercurrentwise" into the storage container 19 without disrupting the operation of the porous wall of the enclosure 12, which is then not subject to any loss of priming.
On the other hand, because the evaporator from FIG. 4 incorporates its own pressurizer storage container 19, it becomes virtually impossible to dispose a plurality of such parallel evaporators in a loop like that of FIG. 1, any pressure imbalance between two reservoirs emptying one to fill the other. Because of this the power that can be conveyed by the loop remains limited.
FIG. 5 is a schematic representation of another type of evaporator as described in the document "Test results of reliable and very high capillary multi-evaporation condensers loops" by S. Van Ost, M. Dubois and G. Beckaert, ICES conference, San Diego, Calif., USA, 1995. Evaporators of the above type are sold by the Belgian company SABCA.
The evaporator is placed in one branch of a circuit that includes one evaporator per branch, a common pressurizer storage container 8 feeding all the branches. Like the previous ones, the evaporator includes a jacket 9 and a porous wall enclosure 12. The reservoir 8 and the evaporator are connected by a tubular pipe lined with a "capillary coupling" 21 consisting of a woven metal tube. In normal operation the heat-conducting liquid reaching the condenser 2 passes through the pressurizer storage container 8 and fills all of the pipe 3 and the space inside the enclosure 12.
With incondensible gas in the loop but with no generation of vapor in the heart of the evaporator, a situation characteristic of operation at high thermal power (typically greater than 50 W for ammonia), the incondensible gas accumulates in the enclosure 12 of the evaporator inside the capillary coupling 21 only. The porous material of the enclosure 12 then continues to be supplied with the heat-conducting liquid, which assures operation of the evaporator.
In the presence of incondensible gas and with generation of vapor in the enclosure 12, a situation characteristic of operation at low thermal power, the vapor that forms in the enclosure can, if the generating pressure is sufficiently high, return into the pressurizer storage container 8, as shown diagrammatically in FIG. 5, and entrain the incondensible gas. The liquid flows at the periphery of the capillary coupling 21 and feeds the porous material of the enclosure, which assures the operation of the evaporator.
It is then possible to place a plurality of evaporators in parallel and the resulting loop is highly resistant to the presence of incondensible gas or vapor in the porous enclosure 12 of the evaporators.
On the other hand, the capillary coupling 21 present in the evaporator feed pipes 3 make the latter rigid and bulky (diameter in the order of 10 mm), drawbacks which can become unacceptable when the loop must be disposed in a restricted space of complex shape, as is often the case in spacecraft, for example.